Open graph fully absolute urls support

- Fixes the issue https://github.com/hexojs/hexo/issues/942
- Allows using fully absolute urls for og image tags (which is mandatory for Facebook).
- Fixes [this peace of code](https://github.com/hexojs/hexo/blob/master/lib/plugins/filter/external_link.js#L25)

I don't quite understand why the optional subdirectory had to be added on both `config.url`and `config.root`.
I believe this is more relevant now.